1. Field of the Invention
This disclosure is generally related to nacelles for aircraft turbofan engines and, more particularly, to nacelles having variable-area fan nozzle (VAFN) assemblies with circumferential cascade ports for fan bypass air.
2. Description of the Related Art
Modern aircraft employ turbofan variants of gas turbine engines that have a low fan pressure ratio (FPR) and high bypass ratio (BPR) for a bypass duct of the engine. As turbofan engine technology has matured, designers have stretched the associated technologies to minimize weight, cost, and maintenance while maximizing efficiency. Because of the extreme limits to which these engines are designed, and the valuable commercial interest in having the most efficient product available, even incremental improvements are sought after in the design of commercial turbofan engines.
Turbofan engines are often optimized for cruise at around 30,000 feet above mean sea level (MSL), an altitude at which many commercial airplanes spend much of their time. Engine geometries that are best suited for cruise at altitude are not necessarily the best geometries suited for other realms of flight, including the different realms of take-off and landing. Environmental parameters such as ambient air pressure, temperature, humidity, kinematic viscosity, Mach speed, and other atmospheric conditions are different as between operation for landing and operation at altitude. Engine power, flow velocity, and Reynolds numbers are different among the operational regimes of take-off, cruise, and landing. Great efforts have been expended to produce aircraft engines that can alter their geometries for different flight regimes.
A variable area fan nozzle (VAFN) assembly allows a turbofan engine to alter its fan bypass duct geometry. Specifically, a VAFN assembly effectively has an exit passage or throat area where fan bypass air exits the engine, and that is constricted or opened, usually by slight amounts. Typically, adjusting the exit passage area is accomplished by axially moving an aft airfoil end of a VAFN assembly forward or aft in relation to an inner fixed structure (IFS) at the exit end of the engine. As the VAFN aft airfoil is moved aft, more cross-section of the exit passage area becomes available for airflow to exit the engine. Slight amounts of VAFN assembly movement in the axial direction, as little as mere inches of movement, can be quite effective in adjusting the airflow out the engine.
During takeoff and landing, the VAFN assembly is typically moved to increase the exit area for bypass airflow. At cruise operation, the VAFN assembly is typically moved to decrease the exit area for bypass airflow. In different regions of flight and power settings, a VAFN assembly may have different positions or alternate scheduling as compared with takeoff and landing.
There is a need in the art for reliable devices to alter engine geometries, including improvements to VAFN designs.